Chapter-8 Sonic Logs

By

Dr. Jorge Salgado Gomes

3/4/2013 1 Chap -8 Duration of this chapter: 2 classes (180’)

R1 R2

R3 R4

Upper Transmitor T1

T2

Well

Wireline cable

Lower Transmitor

Educational Outcomes

• Review some concepts of sound wave propagation & acoustic property of rocks

• Review the principle of the Sonic Tools – Acoustic wave-train; pulse detection; cycle skipping;

calibration; long-spacing tools

• Compare tools – Vertical resolution and depth of investigation

• The use of the Sonic Tool

• Examples of application

3/4/2013 Chap -8 2

Sonic Logging: Principle

• In a homogeneous formation, the acoustic (sound) wave emitted from a source (transmitter) in the middle of the well propagates radially through the mud and a receiver (detector) at a certain distance will record the time the sound waves propagate through the medium.

• The velocity of the medium can therefore be computed.

• Two types of sound waves are propagated: compressional (pressure) waves and shear (distortional) waves.

• Two other types of waves are propagated: The Rayleigh wave and the Stoneley wave.

• At a receiver we have the combination of these various waves arrivals.

• The aim of the tool is to measure the time of propagation of a sound wave through the formation over a fixed distance.

• Basically we need a transmitter and a receiver some distance away on the tool.

3/4/2013 3 Chap -8

Evolution of Sonic Tools

Single Receiver Tool Two Receiver Tool BHC Tool

3/4/2013 4 Chap -8

R1

R2

R3

R4

Upper Transmitor

T1

T2

Well

Wireline cable

Lower Transmitor

BHC (Sonic) Tool

Notes: • However, there are possible

problems with only one T-R pair. For example, large cavings will distort the sonic signal and the tool may read the mud velocity.

• Thus we must use a two receiver setup. But this setup is still subject to sonde tilt and to eliminate this problem, a double inverted system, with 2T and 4R, should be used instead (see diagram).

• Hence the tool name BHC – Bore Hole Compensated.

3/4/2013 5 Chap -8

• Compressional Waves: The wave is propagated in the “compressional mode” that is, the direction of propagation is parallel to the direction of particle displacement. These waves can be propagated through gases, liquids and solids.

• Shear Waves: Are propagated in the shear mode, i.e., the direction of propagation is perpendicular to the direction of particle displacement. Shear waves can be propagated in solids but not in liquids or gases.

• The Rayleigh Wave: It takes place at the interface mud-formation and its speed is quite close to the shear wave speed.

• The Stoneley Wave: It takes place in the mud by interaction between the mud and the formation; it is very sensitive to the wall rigidity. The energy is propagated at a low frequency with small attenuation. Its velocity is lower than the mud velocity.

3/4/2013 Chap -8 6

Sound Waves

Sonic Wave Velocities

Velocity of Compressional Wave Velocity of Shear Wave

3/4/2013 Chap -8 7

2/1

33.1

KVp

2/1

Vs

Where: K is the bulk modulus µ is the shear modulus ρ is the density of the medium

Vs is 1.6 to 2.4 times lower than Vp Velocity of Rayleigh wave ~ 0.9 Vs

Typical BHC Acoustic Log Presentation

• Log readings are scaled not as velocity but as a transit time (also known as slowness), expressed in microseconds per foot.

3/4/2013 8 Chap -8

matrixfluid

matrix

fluidmatrix

tt

tt

ttt

1

from Wyllie’s original publication, reprinted by Ellis 1987

Time Average ft/sec (m/s) Matrix 19500 (5944) Fluid 5000 (1524)

10

15

35

20

25

30

5

0

Poro

sity (

pu)

120 80 70 60 50 Slowness µs/ft

100 90 110

Sonic Response: Wyllie Linear Relationship

Valid for: •Uniform intergranular Ø •Water-bearing formation •Clean formation: no shale •Compacted formation

3/4/2013 9 Chap -8

Wyllie Time Average for SONIC

3/4/2013 10 Chap -8

Tixier’s correction for compaction. Bcp is a compaction Factor: Bcp= Δtsh/100

Raymer-Hunt Equation

3/4/2013 11 Chap -8

Factors Affecting the Measurements

• Compaction: The elastic properties of the rock are regarded as constant if the pressure on the rock is sufficiently large (several thousand psi). At lower pressures (shallow depth), a correction factor may be needed, the compaction factor, Cp.

• Shaliness: If shale is present in the formation, then it will contribute to the measured transit time (Δt).

• Hydrocarbon: No significant effect on Δt • Fractures & vugs: Sonic ignores them (secondary porosity). Sonic Ø

< true total Ø; Secondary Porosity Index (SPI )= Ø – Øs

• Borehole Effects: Should be filled with liquid and hole ˂ 50% bit size. If not we have attenuation of the far receiver signals - “cycle skipping”

3/4/2013 Chap -8 12

shaleshaleshmatrixfluid VtVttt .)1.(.log

Sonic Log Cycle Skipping

3/4/2013 13 Chap -8

Schematic Array Sonic

This tool records the complete waveforms to extract the compressional, shear and stoneley slowness.

3/4/2013 14 Chap -8

Carbonates: Velocity vs. porosity

Data from: Eberli, G.P., Baechle, G.T., Anselmetti, F.S., 2003, Factors controlling elastic properties in

carbonate sediments and rocks, The Leading Edge, July 2003, 654-660

Different pore types cluster

in the porosity-velocity

field, indicating that

scattering at equal porosity

is caused by the

specific pore type and their

resultant elastic property.

microporosity

interparticle

crystalline porosity

densely cemented

low porous

8 MPa effective pressure

0 10 20 30 40 50 60

porosity

7000

vp in m/s

6000

5000

4000

3000

2000

1000

moldic porosity

3/4/2013 15 Chap -8

Chap -8

Material t in µs/m t in µs/ft Ref.

Sandstone 176.5 … 328 (187) 53.8 … 100 (57.0) Se

167.3 … 182 51.0 … 55.5 S

consolidated 172.5 52.6 W

semi-consolidated 182.4 55.6 W

unconsolidated 192.9 58.8 or more W

Limestone 156.2 47.6 W

142.7 … 156.2 43.5 … 47.6 S

149.3 … 155.8 (152.6) 43.5 … 47.6 (46.5) Se

Dolomite 142.7 43.5 W

143.5 43.7 L

142.7 43.5 S

Shale 197 … 558 60 … 170 Se

L - Lehnert et al.1970, S - Schlumberger 1989; Se - Serra 1984; W - Western Atlas 1992

Mean Values for Slowness of Matrix, Pore Filling, Concrete & Casing

3/4/2013 16

Chap -8

Material t in s/m t in s/ft Ref.

Water (pure) 715 218 W

(10 % NaCl) 682 208 W

(20 % NaCl) 620 189 W

Oil 781 238 W

Methane 2034 626 W

Air 2985 910 W

Concrete 273 … 312 (312) 83.3 … 95.1 (95.0) Se

Casing (steel) 187.3 57.1 Se

Mean Values for Slowness of Pore Filling, Concrete & Casing

3/4/2013 17

Examples of Application

• Tools: BHC, Long Space Sonic (LSS), Array Sonic, Dipole Sonic (DSI) • Porosity & Lithology: All tools determine porosity and lithology

– All tools assess secondary porosity – For fracture detection: Array & DSI are the best tools.

• Geomechanical Properties – DSI – Dipole Sonic Imager, for hard & soft rocks – Array Sonic, for hard rocks only

• Seismic Applications – All tools are useful for seismic tie-ins – And for calculation of acoustic impedance for seismic inversion studies – For synthetic seismogram the LSS is a better tool

• GeoSteering horizontal wells – LSS is a better tool to detect boundaries above/below the bit

• Permeability – Can be computed from the Stoneley slowness

• Gas Detection: From the Vp/Vs ratio • Cement Evaluation in Cased Holes

– Cement bond quality based on measurements of acoustic wave attenuation

3/4/2013 Chap -8 18

END

3/4/2013 19 Chap -8

Schematic E2 Signal Detection (Two receivers)

3/4/2013 20 Chap -8